(1) Field of the Invention
The present invention relates to a linking system for attaching a cross tube and a skid of a landing gear, the linking system having a hinge arrangement and being suitable for the dissipation of oscillations derived from the ground resonance phenomenon.
(2) Description of Related Art
Conventionally, a rotorcraft has a landing gear on which the aircraft stands when on the ground. More particularly, skid landing gears are provided with two skids extending parallel to the longitudinal direction of the rotorcraft. The skids are for coming into contact with the ground and they are arranged on either side of the fuselage of the rotorcraft.
Furthermore, skid landing gears are usually provided with cross tubes transversally connecting each of the skids to one another and to the fuselage of the aircraft. The landing gear is thus fastened to the aircraft via the cross tubes.
This type of landing gear is very effective and enables a rotorcraft to land on numerous types of surface.
A rotorcraft having at least three hinged blades may be subjected to a phenomenon of ground resonance.
The oscillations of each blade about its lead-lag axis can become coupled in unstable manner with movements of the fuselage of the rotorcraft that depend on the elastic deformation modes of the landing gear. This is at the origin of the ground resonance phenomenon.
As they rotate, the blades move away from their equilibrium positions and can thus become distributed non-uniformly. This non-uniform distribution of the blades gives rise to unbalance, since the center of gravity of the rotor moves away from the axis of rotation of the rotor. Furthermore, blades that are offset from their equilibrium positions oscillate about those equilibrium positions at an oscillation frequency ωδ. If Ω is the frequency of rotation of the rotor, the fuselage of the rotorcraft is excited at two frequencies |Ω±ωδ|.
When standing on the ground on the landing gear, the rotorcraft fuselage may be thought of as a mass system that is supported by a spring and a damper constituted by the downward branches of the cross tubes. Such system would be characterized by its modes of vibration, especially in roll and in pitching. There is a potential coupling of frequencies when the frequency of the fuselage in roll or in pitching comes close to the frequency of oscillation |Ω+ωδ| or |Ω+ωδ|, either during take-off, when the frequency of the rotor Ω increases, or during landing, when the frequency of the rotor Ω decreases. In practice, only the frequency |Ω+ωδ|, which could be referred to as regressive frequency, supposes a danger of instability when a rotorcraft stands on the ground. In other words, it is the coupling of the frequency of the fuselage with the regressive frequency of oscillation |Ω+ωδ| that can bring about the instability phenomenon known as ground resonance.
In order to avoid such instability, the ground resonance phenomenon can be mitigated by introducing a certain amount of damping in the rotorcraft. There are different options for the location of a damping device—either in the structure of the rotor, like the so called lead-lag dampers, or associated to the fuselage, preferably installed at a landing gear level.
In the case the damper device is associated to the landing gear structure, the adaptation of the landing gear is normally complex. For instance, a compromise needs to be found between the vertical stiffness of the landing gear, which determines the comfort and also the loading imparted to the structure when landing, and the behavior in pitching and in roll when the ground resonance oscillations can occur. Besides, the incorporation of damping parts is often prejudicial for the aerodynamic efficiency, the weight of the rotorcraft and the compactness of the structure.
The design of a skid landing gear is, in consequence, a generally lengthy and difficult process. This design is rarely reviewed during the lifetime of an aircraft.
Some prior art documents describe this type of landing gear dampers intended for alleviating the ground resonance.
Document EP2641831 describes a damping system comprising a torsion bar spring extending in the longitudinal direction of the fuselage and going through two floating bearings and through a fixed bearing attached to the fuselage, and further comprising discrete dampers located between the ends of the torsion bar spring and the fuselage.
Document US 2011/0133378 A1 teaches a damping device suitable for its connection between the cross tube of a landing gear and the fuselage of a helicopter. The device comprises a barrel in turn having a cavity such that a compression member can move inside the cavity defining one hydraulic cavity at each side of the piston, one of them having a hydraulic fluid and a disc of springs, and the other having an additional spring that controls the linear movement of the piston relative to the barrel.
U.S. Pat. No. 3,716,208 discloses a landing gear having a system for dissipating energy located within the structure of the landing gear. The dissipation takes place thanks to liquid springs having one end connected to a skid and another one connected to a crank in turn linked to the cross tube.
Other prior art documents disclose vibration absorbers for the main excitation frequencies of a helicopter—they are thus not specifically designed for dealing with the instability derived from the ground resonance. For instance, document CA2793576 A1 discloses the arrangement of a spring mass system mounted on the landing gear, located at its antinodes and tuned to the helicopter's main excitation frequency.